The purpose of this project is to determine the effect of strengthening exercises on lymphocyte subpopulations (CD 4 T cells, CD 8 T cells, Natural Killer Cells) in healthy adults. There is a decrease in immunocompetence with aging. Specifically, there is a decline in lymphocyte proliferation in response to antigen stimulation in older adults. This phenomenon in part, contributes to the increased susceptibility of the elderly to infection. It is widely believed that regularly performed exercise helps improve resistance to infection. As a replicable and quantifiable stressor, the effect of exercise on immune function has interested numerous investigators. Indeed, many studies have looked at the relationship between aerobic types of exercise and immune function. To our knowledge, this is the first study to look at the effects of strengthening exercises on immune function. Ten subjects have been recruited from the Clinical Center to participate in the first of four exercise groups. Baseline levels of lymphocytes and baseline strength of both quadriceps and hamstrings have been measured and recorded. The group has been divided into an exercise group and control group, based on ranked order of total strength. The exercise group will begin the exercise training shortly and this will continue twice a week for eight weeks. Three more groups will follow in identical fashion.